The Rooneys are one of the few NFL owners who's wealth has come basically just from their football team.
Yes, Art III is a lawyer, but most of his wealth has come from being in the Rooney ownership of the Steelers. His father's wealth has come just from owning the Steelers.
Other owners like Jerry Jones, Dan Snyder, Paul Allen, Stephen Ross, Robert Kraft....they all obtained their wealth outside of football and then they bought into NFL ownership.
I just noticed that the owners at the bottom of that list not only just won the most recent Super Bowl, they are also mostly the ones who's wealth has come via ownership of an NFL franchise (Mara/Tisch, Giants; Al Davis, Raiders; Halas McCaskey, Bears; Dan Rooney, Steelers).